The invention relates to a projection-lens system for projecting a magnified image of a scene reproduced by means of a reproduction element onto a projection screen. The lens system comprises, in this order and viewed from the image side, a first group of lenses, of which at least one outer surface is aspheric, a positive second or main group, of which at least the outer surface which faces the object side is aspheric, and a third group comprising a negative lens whose surface which faces the image side is concave and aspheric. The elements of the first group and the third group are made of a transparent plastic. The invention also relates to a colour-television projection system comprising at least one projection-lens system of this kind.
U.S. Pat. No. 4,348,081 discloses such a projection-lens system intended for projection a scene in one colour on a projection screen, where three monochrome images are super-imposed to form a colour picture, for example a colour-television picture. The third group comprises a planoconcave lens, also referred to as field-curvature correction lens or "field flattener", which compensates for the Petzval curvature of the two other lens groups. The main group may be a single biconvex lens or may comprise two spaced single positive lenses. The lens system is not achromatic. Moreover, in particular in the versions comprising three lenses, the modulation transfer function in the field corners is minimal and the luminance is not satisfactory.
In those versions of the projection-lens system in accordance with U.S. Pat. No. 4,348,081 in which the main group comprises at least one aspheric surface all the lens elements are transparent plastic. The entire projection-lens system is then comparatively light in weight but on account of the substantial temperature dependence of the refractive indices of the relevant the focal length of such a system will vary strongly with the temperature. U.S. Pat. No. 4,348,081 also described a system with a main group having one glass lens. This lens does not have any refractive surface with an aspheric profile, so that the projection-lens system does not provide a satisfactory correction for aberrations. Forming aspheric surfaces on glass lenses is time-consuming and expensive.
In the system comprising a glass main lens a part of the total power of the projection-lens system is provided by the first group and the third group, which are made of a transparent plastic whose refractive index varies with the temperature, so that the focal length of the entire system is still temperature dependent.